The present invention relates to a ferritic steel sheet, which will be referred to as a steel sheet or a thin steel sheet hereinafter, used for making parts for automobile use, the shape fixability in bending of which is excellent due to the development of the {100} texture. Also, the present invention relates to a method for producing the ferritic steel sheet.
In order to reduce the quantity of carbonic acid gas discharged from automobiles, studies to decrease the weight of an automobile, by using high-strength steel sheets for automobile bodies, have continued. Further, in order to ensure the safety of passengers in the automobiles, not only mild steel sheets but also high-strength steel sheets are used in an automobile body. In order to further decrease the weight of an automobile body, there is an increasing demand for enhancing the strength of the high-strength steel sheets to be used for automobile bodies. However, when high-strength steel sheets are subjected to bending, a shape formed by a die in bending tends to return to its initial shape departing from the shape of the die because of the high-strength of the steel sheets. This phenomenon, in which the shape formed by the die in the bending returns to its initial shape, is referred to as spring-back. When this phenomenon of spring-back occurs, it is impossible to obtain a target shape of the part.
For the above reasons, when the conventional automobile bodies are made, only the high-strength steel sheets, the strength of which is not more than 440 MPa, have been used. Although it is necessary to decrease the weight of the automobile bodies by using the high-strength steel sheet, the strength of which is not less than 490 MPa, it is impossible to obtain high-strength steel sheets having excellent shape fixability, that is, it is impossible to obtain high-strength steel sheets on which the phenomenon of spring-back does not occur. Of course, enhancing the shape fixability, by which the shape can be kept after the completion of bending, of high-strength steel sheets and mild steel sheets, the strength of which is not more than 440 MPa, is very important for enhancing the shape accuracy of products such as automobile bodies and electric appliance bodies.
In JP-A-10-72644, there is disclosed a cold-rolled austenitic stainless steel sheet, the quantity of spring-back of which is small, characterized in that the integrated intensity of the {200} texture on a face parallel with a rolling face is not less than 1.5.
This cold-rolled austenitic stainless steel sheet is produced as follows. There is provided a continuous-cast slab, an equiaxed crystal ratio of which is not less than 30%, containing: 0.01 to 0.1 wt % of C, 0.05 to 3.0 wt % of Si, 0.05 to 2.0 wt % of Mn, not more than 0.04 wt % of P, not more than 0.03 wt % of S, not more than 0.1 wt % of Al, 15 to 25 wt % of Cr, 5 to 15 wt % of Ni, 0.005 to 0.3 wt % of N, not more than 0.007 wt % of O, the balance being Fe and inevitable impurities, or alternatively there is provided a continuous-cast slab, an equiaxed crystal ratio of which is not less than 30%, containing: 0.01 to 0.1 wt % of C, 0.05 to 3.0 wt % of Si, 0.05 to 2.0 wt % of Mn, not more than 0.04 wt % of P, not more than 0.03 wt % of S, not more than 0.1 wt % of Al, 15 to 25 wt % of Cr, 5 to 15 wt % of Ni, 0.005 to 0.3 wt % of N, not more than 0.007 wt % of O, optionally containing one of or at least two of: 0.05 to 5.0 wt % of Cu, 0.05 to 5.0 wt % of Co, 0.05 to 5.0 wt % of Mo, 0.05 to 5.0 wt % of W, 0.01 to 0.5 wt % of Ti, 0.01 to 0.5 wt % of Nb, 0.01 to 0.5 wt % of V, 0.01 to 0.5 wt % of Zr, 0.001 to 0.1 wt % of REM, 0.001 to 0.5 wt % of Y, 0.0003 to 0.01 wt % of B, and 0.0003 to 0.01 wt % of Ca, the balance being Fe and inevitable impurities. This continuous-cast slab is heated, rough-hot-rolled, finish-hot-rolled in which the finish rolling temperature at the final rolling pass is not less than 1050xc2x0 C. and the rolling reduction is not less than 15%, annealed appropriately so that the hot-rolled steel sheets can be annealed, and then cold-rolled and annealed so that the cold-rolled steel sheets can be subjected to finish annealing. Due to the foregoing, the cold-rolled austenitic stainless steel sheet is produced without an increase in the crystal grain size.
However, the above cold-rolled austenitic stainless steel sheet is not used for parts of an automobile but used for a bath tubs, pans, tableware and sinks formed by press forming. Further, in the above patent publication of JP-A-10-72644, there are no descriptions about the decrease in a quantity of spring-back of the ferritic steel sheet.
Under the present conditions, when mild steel sheets and high-strength steel sheets are subjected to bending, a large quantity of spring-back is caused, depending upon the strength of the steel sheets, so that the shape fixability of the thus formed parts is deteriorated. The present invention has been accomplished to solve the above problems advantageously. It is an object of the present invention to provide a thin ferritic steel sheet, the shape fixability of which is excellent, and also it is an object of the present invention to provide a method of producing the thin ferritic steel sheet.
According to conventional knowledge, decreasing the yield point of a steel sheet is most important to suppress the occurrence of spring-back on the steel sheet. In order to use a steel sheet having a low yield point, a steel sheet having a low tensile strength must be used. However, the above countermeasure is not sufficient for enhancing the bending formability of the steel sheet and suppressing the quantity of spring-back.
In order to enhance the bending formability so that the problem of the occurrence of spring-back can be fundamentally solved, the present inventors paid attention to a phenomenon in which the texture of a steel sheet has influence on the bending formability, and made investigation into the action and effect in detail. The present inventors tried to find an appropriate material index which corresponds to the bending formability of a steel sheet. As a result of the investigation, the present inventors made the following clear. When a ratio of a {100} plane, which is parallel with a sheet face, to a {111} plane is not less than 1.0 in the texture of a steel sheet, the bending formability of the steel sheet can be improved.
In this connection, it can be assumed that a quantity of presence of the crystal plane parallel with the surface of a thin steel sheet is proportional to a quantity of diffraction of X-ray. Therefore, the quantity of presence of the crystal plane parallel with the surface of a thin steel sheet is found by measuring the X-ray diffraction intensities of the {200} and the {222} plane. Accordingly, the X-ray diffraction intensity on a {200} plane and that on a {222} plane respectively correspond to the quantity of presence of {100} planes and that of {111} planes. Of course, it is possible to say that the ratio of X-ray diffraction intensity {200}/{222}, is equal to the ratio of X-ray diffraction intensity, {100}/{111}, both the {100} plane and the {111} plane of which exist as crystal planes.
The present invention has been accomplished on the basis of the above knowledge. The thin ferritic steel sheet of the present invention is summarized as described in the following items (1) to (10).
(1) A thin ferritic steel sheet having an excellent shape fixability characterized in that a ratio of presence of {100} planes parallel with sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(2) A thin ferritic steel sheet having an excellent shape fixability, comprising: at least 0.027 to less than 0.05 mass % of C, 0.01 to 1.0 mass % of Si, 0.01 to 2.0 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 0.1 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(3) A thin ferritic steel sheet having an excellent shape fixability, comprising: at least 0.027 to less than 0.05 mass % of C, 0.01 to 1.0 mass % of Si, 0.01 to 2.0 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 0.1 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, further containing one of or at least two of not more than 0.2 mass % of Ti, not more than 0.2 mass % of Nb and not more than 0.005 mass % of B, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(4) A thin ferritic steel sheet having an excellent shape fixability, comprising: at least 0.027 to less than 0.05 mass % of C, 0.01 to 1.0 mass % of Si, 0.01 to 2.0 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 0.1 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, further containing one of or at least two of not more than 1.0 mass % of Mo, not more than 2.0 mass % of Cu and not more than 1.0 mass % of Ni, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(5) A thin ferritic steel sheet having an excellent shape fixability, comprising: at least 0.027 to less than 0.05 mass % of C, 0.01 to 1.0 mass % of Si, 0.01 to 2.0 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 0.1 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, further containing one of or at least two of not more than 0.2 mass % of Ti, not more than 0.2 mass % of Nb and not more than 0.005 mass % of B, furthermore containing one of or at least two of not more than 1.0 mass % of Mo, not more than 2.0 mass % of Cu and not more than 1.0 mass % of Ni, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(6) A thin ferritic steel sheet having an excellent shape fixability, comprising: 0.05 to 0.25 mass % of C, 0.01 to 2.5 mass % of Si, 0.01 to 2.5 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 1.0 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(7) A thin ferritic steel sheet having an excellent shape fixability, comprising: 0.05 to 0.25 mass % of C, 0.01 to 2.5 mass % of Si, 0.01 to 2.5 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 1.0 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, further containing one of or at least two of not more than 0.2 mass % of Ti, not more than 0.2 mass % of Nb, not more than 0.2 mass % of V, not more than 1.0 mass % of Cr and not more than 0.005 mass % of B, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(8) A thin ferritic steel sheet having an excellent shape fixability, comprising: 0.05 to 0.25 mass % of C, 0.01 to 2.5 mass % of Si, 0.01 to 2.5 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 1.0 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, further containing one of or at least two of not more than 1.0 mass % of Mo, not more than 2.0 mass % of Cu and not more than 1.0 mass % of Ni, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(9) A thin ferritic steel sheet having an excellent shape fixability, comprising: 0.05 to 0.25 mass % of C, 0.01 to 2.5 mass % of Si, 0.01 to 2.5 mass % of Mn, not more than 0.15 mass % of P, not more than 0.03 mass % of S, 0.01 to 1.0 mass % of Al, not more than 0.01 mass % of N, not more than 0.007 mass % of O, further containing one of or at least two of not more than 0.2 mass % of Ti, not more than 0.2 mass % of Nb, not more than 0.2 mass % of V, not more than 1.0 mass % of Cr and not more than 0.005 mass % of B, furthermore containing one of or at least two of not more than 1.0 mass % of Mo, not more than 2.0 mass % of Cu and not more than 1.0 mass % of Ni, the balance being Fe and inevitable impurities, wherein a ratio of presence of {100} planes parallel with a sheet surface to {111} planes is not less than 1.0, and TSxc3x97El, which represents a product of maximum tensile strength (TS) multiplied by rupture elongation (El) of the steel sheet, is at least 13,860 MPa %.
(10) A thin ferritic steel sheet having an excellent shape fixability, according to one of items (1) to (9), wherein the sheet surface is plated.
The method of producing a thin ferritic steel sheet having an excellent shape fixability of the present invention is described in the following items (11) to (18).
(11) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to one of the items (1) to (9), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at a temperature range from a temperature not higher than 950xc2x0 C. to a temperature not lower than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher than 950xc2x0 C.; completing the hot rolling at a temperature not lower than transformation temperature Ar3; cooling the hot-rolled steel strip; and coiling the hot-rolled steel strip at a temperature not higher than critical temperature T0 determined by the following expression.             T      0        =                            -          650.4                xc3x97        C        ⁢        %            -              50.6        xc3x97        Mneq            +      894.3        where                              Mneq          =                      xe2x80x83                    ⁢                                    Mn              ⁢              %                        +                          0.5              xc3x97              Ni              ⁢              %                        -                          1.49              xc3x97              Si              ⁢              %                        -                          1.05              xc3x97                                                                                xe2x80x83                    ⁢                                    Mo              ⁢              %                        -                          0.44              xc3x97              W              ⁢              %                        +                          0.37              xc3x97              Cr              ⁢              %                        +                          0.67              xc3x97                                                                                xe2x80x83                    ⁢                                    Cu              ⁢              %                        -                          23              xc3x97              P              ⁢              %                        +                          13              xc3x97              Al              ⁢              %                                          
(12) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to item (10), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at temperature range from a temperature not higher than 950xc2x0 C. to a temperature not lower than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher than 950xc2x0 C.; completing the hot rolling at a temperature not lower than transformation temperature Ar3; cooling the hot-rolled steel strip; coiling the hot-rolled steel strip at a temperature not higher than critical temperature T0 determined by the following expression; and plating on the hot-rolled steel strip.             T      0        =                            -          650.4                xc3x97        C        ⁢        %            -              50.6        xc3x97        Mneq            +      894.3        where                              Mneq          =                      xe2x80x83                    ⁢                                    Mn              ⁢              %                        +                          0.5              xc3x97              Ni              ⁢              %                        -                          1.49              xc3x97              Si              ⁢              %                        -                          1.05              xc3x97                                                                                xe2x80x83                    ⁢                                    Mo              ⁢              %                        -                          0.44              xc3x97              W              ⁢              %                        +                          0.37              xc3x97              Cr              ⁢              %                        +                          0.67              xc3x97                                                                                xe2x80x83                    ⁢                                    Cu              ⁢              %                        -                          23              xc3x97              P              ⁢              %                        +                          13              xc3x97              Al              ⁢              %                                          
(13) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to one of the items (1) to (9), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at a temperature not higher than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher than the Ar3; cooling the hot-rolled steel strip; and coiling the hot-rolled strip or additionally recovering and recrystallizing the hot-rolled steel strip.
(14) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to item (10), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at a temperature not higher than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher that the Ar3; cooling the hot-rolled steel strip; coiling the hot-rolled steel strip or additionally recovering and recrystallizing the hot-rolled steel strip; and plating on the hot-rolled steel strip.
(15) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to one of the items (1) to (9), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at a temperature range from a temperature not higher than 950xc2x0 C. to a temperature not lower than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher than 950xc2x0 C.; completing the hot rolling at a temperature not lower than transformation temperature Ar3; cooling the hot-rolled steel strip; coiling the hot-rolled steel strip at a temperature not higher than critical temperature T0 determined by the following expression; pickling the hot-rolled steel strip; conducting cold-rolling on the steel strip at a rolling reduction lower than 80%; heating the cold-rolled steel strip in a temperature range from a temperature not lower than 600xc2x0 C. to a temperature lower than transformation temperature Ac3; and cooling the steel strip.             T      0        =                            -          650.4                xc3x97        C        ⁢        %            -              50.6        xc3x97        Mneq            +      894.3        where                              Mneq          =                      xe2x80x83                    ⁢                                    Mn              ⁢              %                        +                          0.5              xc3x97              Ni              ⁢              %                        -                          1.49              xc3x97              Si              ⁢              %                        -                          1.05              xc3x97                                                                                xe2x80x83                    ⁢                                    Mo              ⁢              %                        -                          0.44              xc3x97              W              ⁢              %                        +                          0.37              xc3x97              Cr              ⁢              %                        +                          0.67              xc3x97                                                                                xe2x80x83                    ⁢                                    Cu              ⁢              %                        -                          23              xc3x97              P              ⁢              %                        +                          13              xc3x97              Al              ⁢              %                                          
(16) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to item (10), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at a temperature range from a temperature not higher than 950xc2x0 C. to a temperature not lower than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher than 950xc2x0 C.;
completing the hot rolling at a temperature not lower than transformation temperature Ar3; cooling the hot-rolled steel strip; coiling the hot-rolled steel strip at a temperature not higher than critical temperature T0 determined by the following expression; pickling the hot-rolled steel strip; conducting cold-rolling on the steel strip at a rolling reduction lower than 80%; heating the cold-rolled steel strip in a temperature range from a temperature not lower than 600xc2x0 C. to a temperature lower than transformation temperature AC3; cooling the steel strip; and plating on the steel strip.             T      0        =                            -          650.4                xc3x97        C        ⁢        %            -              50.6        xc3x97        Mneq            +      894.3        where                              Mneq          =                      xe2x80x83                    ⁢                                    Mn              ⁢              %                        +                          0.5              xc3x97              Ni              ⁢              %                        -                          1.49              xc3x97              Si              ⁢              %                        -                          1.05              xc3x97                                                                                xe2x80x83                    ⁢                                    Mo              ⁢              %                        -                          0.44              xc3x97              W              ⁢              %                        +                          0.37              xc3x97              Cr              ⁢              %                        +                          0.67              xc3x97                                                                                xe2x80x83                    ⁢                                    Cu              ⁢              %                        -                          23              xc3x97              P              ⁢              %                        +                          13              xc3x97              Al              ⁢              %                                          
(17) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to one of the items (1) to (9), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at a temperature not higher than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher than the Ar3; cooling the hot-rolled steel strip; and coiling the hot-rolling steel strip or additionally recovering and recrystallizing the hot-rolling steel strip; pickling the hot-rolled steel strip; conducting cold-rolling on the steel strip at a rolling reduction lower than 80%; heating the cold-rolled steel strip in a temperature range from a temperature not lower than 600xc2x0 C. to a temperature lower than transformation temperature Ac3; and cooling the steel strip.
(18) A method of producing a thin ferritic steel sheet having an excellent shape fixability according to item (10), comprising the steps of: conducting hot-rolling on a slab of a predetermined composition so that a total rolling reduction of 25% or more in the hot rolling conducted at a temperature not higher than transformation temperature Ar3 and a coefficient of friction of 0.2 or less in the hot rolling conducted at a temperature not higher than transformation temperature Ar3; cooling the hot-rolled steel strip; and coiling the hot-rolled steel strip or additionally recovering and recrystallizing the hot-rolled steel strip; pickling the hot-rolled steel strip; conducting cold-rolling on the steel strip at a rolling reduction lower than 80%; heating the cold-rolled steel strip in a temperature range from a temperature not lower than 600xc2x0 C. to a temperature lower than transformation temperature Ac3; cooling the steel strip; and plating on the steel strip.